[PATCH v2 5/5] shmem: stable directory cookies

[Chuck Lever <cel@kernel.org>]

From: Chuck Lever <chuck.lever@oracle.com>

The current cursor-based directory cookie mechanism doesn't work
when a tmpfs filesystem is exported via NFS. This is because NFS
clients do not open directories. Each server-side READDIR operation
has to open the directory, read it, then close it. The cursor state
for that directory, being associated strictly with the opened
struct file, is discarded after each READDIR operation.

Directory cookies are cached not only by NFS clients, but also by
user space libraries on those clients. Essentially there is no way
to invalidate those caches when directory offsets have changed on
an NFS server after the offset-to-dentry mapping changes. Thus the
whole application stack depends on unchanging directory cookies.

The solution we've come up with is to make the directory cookie for
each file in a tmpfs filesystem stable for the life of the directory
entry it represents.

Add a per-directory xarray. shmem_readdir() uses this to map each
directory offset (an loff_t integer) to the memory address of a
struct dentry.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 mm/shmem.c |  202 +++++++++++++++++++++++++++++++-----------------------------
 1 file changed, 106 insertions(+), 96 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 733b98ca8517..35eb2f1368dd 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2939,6 +2939,55 @@ static struct xarray *shmem_doff_map(struct inode *dir)
 	return &SHMEM_I(dir)->doff_map;
 }
 
+static int shmem_doff_add(struct inode *dir, struct dentry *dentry)
+{
+	struct shmem_inode_info *info = SHMEM_I(dir);
+	struct xa_limit limit = XA_LIMIT(2, U32_MAX);
+	u32 offset;
+	int ret;
+
+	if (dentry->d_fsdata)
+		return -EBUSY;
+
+	offset = 0;
+	ret = xa_alloc_cyclic(shmem_doff_map(dir), &offset, dentry, limit,
+			      &info->next_doff, GFP_KERNEL);
+	if (ret < 0)
+		return ret;
+
+	dentry->d_fsdata = (void *)(unsigned long)offset;
+	return 0;
+}
+
+static struct dentry *shmem_doff_find_after(struct dentry *dir,
+					    unsigned long *offset)
+{
+	struct xarray *xa = shmem_doff_map(d_inode(dir));
+	struct dentry *d, *found = NULL;
+
+	spin_lock(&dir->d_lock);
+	d = xa_find_after(xa, offset, ULONG_MAX, XA_PRESENT);
+	if (d) {
+		spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
+		if (simple_positive(d))
+			found = dget_dlock(d);
+		spin_unlock(&d->d_lock);
+	}
+	spin_unlock(&dir->d_lock);
+	return found;
+}
+
+static void shmem_doff_remove(struct inode *dir, struct dentry *dentry)
+{
+	u32 offset = (u32)(unsigned long)dentry->d_fsdata;
+
+	if (!offset)
+		return;
+
+	xa_erase(shmem_doff_map(dir), offset);
+	dentry->d_fsdata = NULL;
+}
+
 /*
  * During fs teardown (eg. umount), a directory's doff_map might still
  * contain entries. xa_destroy() cleans out anything that remains.
@@ -2971,6 +3020,10 @@ shmem_mknod(struct mnt_idmap *idmap, struct inode *dir,
 		if (error && error != -EOPNOTSUPP)
 			goto out_iput;
 
+		error = shmem_doff_add(dir, dentry);
+		if (error)
+			goto out_iput;
+
 		error = 0;
 		dir->i_size += BOGO_DIRENT_SIZE;
 		dir->i_ctime = dir->i_mtime = current_time(dir);
@@ -3048,6 +3101,10 @@ static int shmem_link(struct dentry *old_dentry, struct inode *dir, struct dentr
 			goto out;
 	}
 
+	ret = shmem_doff_add(dir, dentry);
+	if (ret)
+		goto out;
+
 	dir->i_size += BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
@@ -3066,6 +3123,8 @@ static int shmem_unlink(struct inode *dir, struct dentry *dentry)
 	if (inode->i_nlink > 1 && !S_ISDIR(inode->i_mode))
 		shmem_free_inode(inode->i_sb);
 
+	shmem_doff_remove(dir, dentry);
+
 	dir->i_size -= BOGO_DIRENT_SIZE;
 	inode->i_ctime = dir->i_ctime = dir->i_mtime = current_time(inode);
 	inode_inc_iversion(dir);
@@ -3124,24 +3183,37 @@ static int shmem_rename2(struct mnt_idmap *idmap,
 {
 	struct inode *inode = d_inode(old_dentry);
 	int they_are_dirs = S_ISDIR(inode->i_mode);
+	int error;
 
 	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE | RENAME_WHITEOUT))
 		return -EINVAL;
 
-	if (flags & RENAME_EXCHANGE)
+	if (flags & RENAME_EXCHANGE) {
+		shmem_doff_remove(old_dir, old_dentry);
+		shmem_doff_remove(new_dir, new_dentry);
+		error = shmem_doff_add(new_dir, old_dentry);
+		if (error)
+			return error;
+		error = shmem_doff_add(old_dir, new_dentry);
+		if (error)
+			return error;
 		return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
+	}
 
 	if (!simple_empty(new_dentry))
 		return -ENOTEMPTY;
 
 	if (flags & RENAME_WHITEOUT) {
-		int error;
-
 		error = shmem_whiteout(idmap, old_dir, old_dentry);
 		if (error)
 			return error;
 	}
 
+	shmem_doff_remove(old_dir, old_dentry);
+	error = shmem_doff_add(new_dir, old_dentry);
+	if (error)
+		return error;
+
 	if (d_really_is_positive(new_dentry)) {
 		(void) shmem_unlink(new_dir, new_dentry);
 		if (they_are_dirs) {
@@ -3206,6 +3278,11 @@ static int shmem_symlink(struct mnt_idmap *idmap, struct inode *dir,
 		folio_unlock(folio);
 		folio_put(folio);
 	}
+
+	error = shmem_doff_add(dir, dentry);
+	if (error)
+		goto out_iput;
+
 	dir->i_size += BOGO_DIRENT_SIZE;
 	dir->i_ctime = dir->i_mtime = current_time(dir);
 	inode_inc_iversion(dir);
@@ -3256,77 +3333,20 @@ static const char *shmem_get_link(struct dentry *dentry,
 	return folio_address(folio);
 }
 
-static struct dentry *scan_positives(struct dentry *cursor,
-					struct list_head *p,
-					loff_t count,
-					struct dentry *last)
-{
-	struct dentry *dentry = cursor->d_parent, *found = NULL;
-
-	spin_lock(&dentry->d_lock);
-	while ((p = p->next) != &dentry->d_subdirs) {
-		struct dentry *d = list_entry(p, struct dentry, d_child);
-		// we must at least skip cursors, to avoid livelocks
-		if (d->d_flags & DCACHE_DENTRY_CURSOR)
-			continue;
-		if (simple_positive(d) && !--count) {
-			spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
-			if (simple_positive(d))
-				found = dget_dlock(d);
-			spin_unlock(&d->d_lock);
-			if (likely(found))
-				break;
-			count = 1;
-		}
-		if (need_resched()) {
-			list_move(&cursor->d_child, p);
-			p = &cursor->d_child;
-			spin_unlock(&dentry->d_lock);
-			cond_resched();
-			spin_lock(&dentry->d_lock);
-		}
-	}
-	spin_unlock(&dentry->d_lock);
-	dput(last);
-	return found;
-}
-
 static loff_t shmem_dir_llseek(struct file *file, loff_t offset, int whence)
 {
-	struct dentry *dentry = file->f_path.dentry;
 	switch (whence) {
-		case 1:
-			offset += file->f_pos;
-			fallthrough;
-		case 0:
-			if (offset >= 0)
-				break;
-			fallthrough;
-		default:
-			return -EINVAL;
-	}
-	if (offset != file->f_pos) {
-		struct dentry *cursor = file->private_data;
-		struct dentry *to = NULL;
-
-		inode_lock_shared(dentry->d_inode);
-
-		if (offset > 2)
-			to = scan_positives(cursor, &dentry->d_subdirs,
-					    offset - 2, NULL);
-		spin_lock(&dentry->d_lock);
-		if (to)
-			list_move(&cursor->d_child, &to->d_child);
-		else
-			list_del_init(&cursor->d_child);
-		spin_unlock(&dentry->d_lock);
-		dput(to);
-
-		file->f_pos = offset;
-
-		inode_unlock_shared(dentry->d_inode);
+	case SEEK_CUR:
+		offset += file->f_pos;
+		fallthrough;
+	case SEEK_SET:
+		if (offset >= 0)
+			break;
+		fallthrough;
+	default:
+		return -EINVAL;
 	}
-	return offset;
+	return vfs_setpos(file, offset, U32_MAX);
 }
 
 static bool shmem_dir_emit(struct dir_context *ctx, struct dentry *dentry)
@@ -3334,7 +3354,7 @@ static bool shmem_dir_emit(struct dir_context *ctx, struct dentry *dentry)
 	struct inode *inode = d_inode(dentry);
 
 	return ctx->actor(ctx, dentry->d_name.name, dentry->d_name.len,
-			  ctx->pos, inode->i_ino,
+			  (loff_t)dentry->d_fsdata, inode->i_ino,
 			  fs_umode_to_dtype(inode->i_mode));
 }
 
@@ -3361,36 +3381,26 @@ static bool shmem_dir_emit(struct dir_context *ctx, struct dentry *dentry)
  */
 static int shmem_readdir(struct file *file, struct dir_context *ctx)
 {
-	struct dentry *dentry = file->f_path.dentry;
-	struct dentry *cursor = file->private_data;
-	struct list_head *anchor = &dentry->d_subdirs;
-	struct dentry *next = NULL;
-	struct list_head *p;
-
-	if (!dir_emit_dots(file, ctx))
-		return 0;
+	struct dentry *dentry, *dir = file->f_path.dentry;
+	unsigned long offset;
 
-	if (ctx->pos == 2)
-		p = anchor;
-	else if (!list_empty(&cursor->d_child))
-		p = &cursor->d_child;
-	else
-		return 0;
+	lockdep_assert_held(&d_inode(dir)->i_rwsem);
 
-	while ((next = scan_positives(cursor, p, 1, next)) != NULL) {
-		if (!shmem_dir_emit(ctx, dentry))
+	if (!dir_emit_dots(file, ctx))
+		goto out;
+	for (offset = ctx->pos - 1; offset < ULONG_MAX - 1;) {
+		dentry = shmem_doff_find_after(dir, &offset);
+		if (!dentry)
 			break;
-		ctx->pos++;
-		p = &next->d_child;
+		if (!shmem_dir_emit(ctx, dentry)) {
+			dput(dentry);
+			break;
+		}
+		ctx->pos = offset + 1;
+		dput(dentry);
 	}
-	spin_lock(&dentry->d_lock);
-	if (next)
-		list_move_tail(&cursor->d_child, &next->d_child);
-	else
-		list_del_init(&cursor->d_child);
-	spin_unlock(&dentry->d_lock);
-	dput(next);
 
+out:
 	return 0;
 }